This invention relates to a heat transfer sheet and, more particularly, to a heat transfer sheet to be used in a sublimation transfer system of the heat-sensitive type, or a heat transfer sheet to be used in an electrothermal transfer system.
In the background art, various heat transfer methods have been known, and among them, there has been proposed a method in which a sublimable dye is used as a recording agent, and is carried on a substrate film such as polyester film to form a heat transfer sheet, and various full color images are formed on a transferable material dyeable with a sublimable dye. For example, an image receiving sheet having a dye receptive layer on paper, plastic film, etc. In this case, thermal heads of a printer are used as the heating means, and a large number of color dots of 3 or 4 colors are transferred onto a transferable material, thereby reproducing the full color image of the original with color dots of multiple colors.
The image thus formed is very sharp and also excellent in transparency, because the colorants used are dyes, and therefore the thus obtained image has excellent reproducibility and gradation of intermediate color. It has been rendered possible to form an image similar to the image obtained by the off-set printing or the gravure printing of the prior art, and also of high quality comparable with full color photographic images.
As the substrate film for the heat transfer sheet of the sublimation transfer type as mentioned above, papers such as condenser paper may be also employed in some cases, but such thin paper has low strength, particularly weak bursting strength, and therefore, it is desirable to use a film of tough plastic such as polyester resin as the substrate film.
However, even in such a case, there will ensue such problem as follows. That is, during printing, high heat of around 250.degree. to 300.degree. C. or higher is applied by the heating heads on the heat transfer sheet, whereby the phenomenon of partial fusion of the substrate film onto the heads occurs and delivery of the heat transfer sheet is obstructed.
This phenomenon is called sticking, which not degrades the sharpness of recording, but also brings about troubles in operation such as defective running of the heat transfer sheet, etc. Also, wrinkles are formed by heat on the substrate film, whereby color slippage of the dye image may also occur. This is particularly liable to be generated when the shade of the printed image is partially imbalanced.
As a measure which enables use of a plastic film as the substrate film of heat transfer sheet, there has been proposed a provision of a heat-resistant protective layer such as a thermosetting resin, etc. on the surface opposite to the dye layer, etc.
However, even by use of these methods, if the heat-resistant protective layer is made thick to an extent effective for prevention of the sticking phenomenon for effecting speed-up of recording, resolution of the printed image is lowered and hence it cannot be still a sufficient solving measure under the present situation.
On the other hand, there also has been known a heat transfer sheet of the electrothermal transfer type. In the heat transfer system as described above by use of a thermal head, since heat efficiency of thermal head is limited, there is the problem that migration of heat migratable dye is insufficient and the problem that high speed printing cannot be done easily. Therefore, there has been developed the electrothermal transfer system as the technique which forms images of high density at high speed. By passing electric current through this type of heat transfer sheet having a resistance layer which is capable of generating heat by electric current and a heat migratable dye layer from electrode heads, heat generation corresponding to image information signals occurs in the resistance layer. As the result of heating of the dye layer with the generated heat, the dye is migrated onto the image receiving layer to form images.
As the electrothermal transfer sheet of the background art, there is one having a resistance layer which is prepared by dispersing electroconductive carbon powder in a binder dissolved in a solvent, coating the dispersion on a film such as polyethylene terephthalate, and having a heat migratable dye layer on the opposite surface of the sheet. In the electrothermal transfer system, since heat generation is effected by use of electrode heads directly on the sheet and internally in the resistance layer, thermal loads onto the resistance layer or the substrate film are great, whereby thermal fusion of the head electrodes with the resistance layer or the substrate film may have sometimes occurred to bring about troubles of running or printed images.
Thus, in the electrothermal transfer recording system, heat generation is carried out by use of electrode heads in the resistance layer directly, so that the temperature of the resistance layer and the substrate film will more readily become higher locally and the thermal load onto the resistance layer or the substrate film is very great as compared with the heat-sensitive sublimation transfer recording system. For this reason, in the background art, the substrate film made of polyethylene terephthalate (PET), etc. has poor mechanical strength during heat generation, so that wrinkling and breaking phenomena have sometimes occurred.
Also, due to friction with the electrode heads and defective adhesion to the substrate sheet, the resistance layer may be cut off, or the resistance layer may be melted by high temperature heat generation. The thus cut scum or the melted product may be attached between the electrode heads, whereby excessive current may flow between the electrodes, resulting in thermal fusion between the current passage type heat transfer sheet and the electrode heads; consequently, such thermal function causes problems of bringing about defective running or troubles in printed images.